In recent years, in the automotive industry, the introduction of apparatuses and systems aimed at the safety of automobiles has been actively promoted. Among these are air bag systems. These are systems which are mounted on an automobile and which, at the time of an automotive collision, expand an air bag with gas or the like between a steering wheel or an instrument panel or the like and a passenger before the passenger impacts therewith, and absorb the kinetic energy of the passenger with the object of reducing injuries thereof.
In the past, air bag systems were of the type employing explosive chemicals, but they are expensive, and due to environmental problems and recycling problems, in recent years, a type has been developed using an accumulator made of a steel pipe filled with argon gas or the like, and this type is increasingly used.
The above-mentioned accumulator is a container which at normal times maintains the gas or the like which is blown into an air bag at the time of the collision of an automobile at a high pressure and which at the time of a collision discharges a high pressure inert gas such as argon gas in a single burst. Accordingly, a steel pipe used as such an accumulator (the steel pipe being hereinafter referred to as a steel pipe for an air bag) is to receive a stress at a high strain rate in an extremely short period of time. Therefore, compared with a simple structure such as a conventional pressure cylinder or a line pipe, the above-described steel pipe is required to have superior dimensional accuracy, workability, and weldability, and it must also have high strength and excellent resistance to bursting.
Taking into consideration use in cold regions, it is also desired that it have sufficient toughness at around −40° C. In recent years, a guarantee of safety has been equally desired in cold regions, and there is a strong demand for such low temperature toughness.
A steel pipe which forms an accumulator is usually obtained by cutting a pipe having an outer diameter of 25-100 mm and a wall thickness of 2-5 mm to the necessary length.
Such a steel pipe for air bags and a process for its manufacture are disclosed in, for example, JP P8-325641A1, JP P10-140238A1, JP P10-140249A1, JP P10-140250A1, and JP P10-140283A1.
Thus, in the past, a steel pipe for an air bag having sufficient performance has been obtained. However, as the trend towards a decrease in the weight of automobiles becomes ever stronger in recent years, there has also come to be a demand for a decrease in the size and weight of air bag apparatuses. Therefore, at present, there has come to be a demand for an increase in the pressure of gas blown into air bags and for a decrease in the wall thickness of steel pipes.
The technologies described in the above-mentioned patent publications are all directed at “a steel pipe having high strength and high toughness for an air bag”, and they have a tensile strength of at least 590 MPa as a goal. However, as is clear from the examples therein, the tensile strength is at most 883 MPa. Therefore, it is thought that as the pressure of the gas blown into air bags increases and as the wall thickness of steel pipes decreases, there may be cases in which the technologies disclosed in the above-mentioned patent publications are not necessarily adequate from the standpoint of resistance to bursting.
In addition, none of the above-mentioned patent publications has any description of low temperature toughness, so further improvements are desired in order to adequately cope with the present-day requirements described above.